adventure: Tier 6 postgame "Mythic" dungeons (P1–P7 calibration)

Five post-game dungeons above the T5 ceiling, gated on both T5 bosses beaten
+ level 18. Opt-in endgame: deadly solo, clearable by a party with Pete + pets.

- P1 gating: postgameUnlocked (T5 clears + level floor), zonesForLevel excludes
  T6 unconditionally; wired into startZoneRun, !zone/!expedition, party accept,
  boredom picker, and the list dividers.
- P2 bestiary: 15 elites + 5 signature bosses (Layer-1 stat blocks).
- P3 zone defs + 4-region registries; ZoneLootEntry.BossOnly.
- P4 five zone graphs on a shared builder (44–52 rooms, no soft-lock; Ossuary
  secret Verse nodes).
- P5 loot: BossOnly enforced; signature items are real registry magic items;
  five Thom pity recipes off the per-zone crafting anchors.
- P6 narration/flavor (5 files), T6 achievements, Pete stays zone-parametric.
- P7 (in progress): sim can now reach gated T6 (SimRunner.SeedPostgameUnlock +
  IsPostgameZone). First calibration pass on millenia — hardened ossuary +
  drowned_star, softened first_hoard + unplace; last_meridian in band.

Fix: party members were refused from every T6 zone because expeditionCmdAccept
ran the level gate (which excludes T6) before the postgame check — the intended
party endgame was unreachable. Route T6 through postgameUnlocked. Regression
tests added.

internal/plugin/magic_items.go

@@ -46,10 +46,11 @@ type MagicItem struct {
 }
 
 // magicItemOverlay — hand-authored magic items that win on ID collision with
-// the generated SRD dump. Empty for now: the classifier output is the starting
-// point, and corrections (or wholly new items) land here rather than being
-// edited into the generated file.
-var magicItemOverlay []MagicItem
+// the generated SRD dump. Corrections (or wholly new items) land here rather
+// than being edited into the generated file. Today it carries the T6 signature
+// items (postgame_magic_items.go); this is a plain var reference, not an init()
+// append, so it resolves before buildMagicItemRegistry() reads it.
+var magicItemOverlay = postgameSignatureMagicItems
 
 // magicItemRegistry is the merged lookup table: the generated SRD dump with the
 // hand-authored overlay layered on top.